You could try Sharpconstruct http://www.icewalkers.com/Linux/Software/522950/SharpConstruct.html Or I beleive there is a test build of Blender with displacement painting.
here is another one: http://mypaint.intilinux.com/?page_id=9 this one is even used for the next Blender Movie "Durian" http://durian.blender.org/about/